High performance, infrared photon detectors are commonly made from the narrow bandgap semiconductor mercury-cadmium-telluride (HgCdTe) which generates electron-hole pairs when struck by infrared radiation. In this material, the bandgap is dependent on the ratio of cadmium to mercury. For example, a detector made from Hg1−xCdxTe with x=0.3 would respond, at a temperature of 80K, to all wavelengths up to 5 μm. In practice a lower limit is set, either intentionally or unintentionally, by the presence of some other component in the optical path. For example, the lower limit could be set by using an optical filter that cut-on at 3 μm so that the combination of filter and detector would then respond to all wavelengths between 3 μm and 5 μm. Such a conventional detector gives a signal proportional to the integrated photon flux in the wavelength band. However, the spectral distribution of emissions from a source can give information about the source and many applications require the ability to image a scene at infrared wavelengths in two or more different spectral bands, a capability commonly called, where two spectral bands are used, “dual colour thermal imaging”. Such applications include rejection of background clutter, target discrimination and remote sensing for temperature determination and pollution monitoring.
Such dual-band HgCdTe detector arrays comprise two separate photovoltaic detectors within each unit cell, one on top of the other. The photodiode with the shorter cut-off wavelength acts as a long-wavelength-pass filter for the longer cut-off photodiode. The use of two spatially coincident detectors that respond in different wavelength bands, the so-called two-colour detector, gives useful information about the source.
There are two principal types of HgCdTe two-colour detectors-the metal-insulator-semiconductor (MIS) heterojunction detector and the triple layer heterojunction diode. The MIS heterojunction includes a thin wide bandgap n-type layer over a thick narrow bandgap n-type layer. The structure can detect radiation consistent with the wide bandgap layer or wide plus narrow bandgap layer, depending upon the voltage across the layers. However this structure requires precise control of both the layer thickness and the carrier concentration. It also only detects narrow and wide bandgap radiation separately.
The triple layer heterojunction diode includes back-to-back n-p-n diodes, one photodiode of long wavelength, LW, the other of mid wavelength, MW, for example. Operated by biasing between two terminals, one bias polarity results in the top (long wavelength, LW) photodiode of the bias-selectable detector being reverse-biased. The photocurrent of the MW photodiode is shunted by the low impedance of the forward-biased MW photodiode and the only photocurrent to emerge in the external circuit is the LW photocurrent. When the bias voltage is reversed, the situation reverses. The LW photodiode is then forward-biased and the MW photodiode is reverse-biased. In this case the LW photocurrent is shunted and only the MW photocurrent is seen in the external circuit. This provides detection in two adjacent wavebands within each unit cell, with the optical areas of the two photodiodes spatially registered and co-located. Such co-location improves the accuracy of any calculation which assumes a single source for the two wavelengths of radiation. Even though the bias-selectable dual-band HgCdTe detector affords spatial co-location of the two detectors, it does not allow temporal simultaneity of detection. Either one or other of the photodiodes is functioning, depending on the bias polarity applied across the back-to-back diode pair. Other problems also arise from the fact that it does not allow independent selection of the optimum bias for each photodiode and that there can be substantial MW cross-talk in the LW detector.
Some applications require simultaneity of detection in the two spectral bands. This has been achieved in an independently accessible two-colour IR detector, which provides independent electrical access to each of two spatially co-located back-to-back photodiodes. The p-n-n-p structure was formed by two Hg1−xCdxTe layers grown sequentially onto a cadmium-zinc-telluride, CdZnTe, substrate.
However, previously available two-colour detectors are responsive in two overlapping wavelength bands. There is a need for two-colour detectors that respond in two non-adjacent wavelength bands, i.e. a detector in which two wavelength bands produce a signal, the two wavelength bands being separated by a wavelength band that does not produce a signal.
In our co-pending UK patent application no. 0412942.5 and international patent application no. PCT/GB2005/050083 there is disclosed an electromagnetic radiation detector that is responsive to two discrete wavelength ranges, thus allowing the response of the detector to be matched to discrete atmospheric transmission windows that are separated by wavelength bands in which infrared radiation does not easily propagate. Complete separation or large spacing of the detection bands leads to an improved ability to characterise the temperature or wavelength of an external source, enabling machine intelligence to make a better assessment of the physical nature of the source. Applications of such a detector include clutter rejection and target identification. The detector described in the UK patent application above comprises a plurality of layers of semiconductor material formed on a substrate substantially transparent to electromagnetic radiation having wavelengths in a desired wavelength range. A first layer, doped to provide a first type of electrical conductivity, has a bandgap selected for absorbing radiation up to a first wavelength; a second layer, doped to provide a second type of electrical conductivity, has a bandgap selected for absorbing radiation up to a second wavelength that is longer than the first wavelength; and a third layer, doped to provide the first type of electrical conductivity, has a bandgap selected for absorbing radiation up a third wavelength that is longer than the second wavelength. Sandwiched between these absorbing layers are thin barrier layers that are doped to provide the same type of electrical conductivity as the second layer but have a bandgap substantially greater than the second layer. The purpose of these barrier layers is to prevent minority carriers generated in the second layer from reaching the junctions; instead they recombine with majority carriers in the second layer. Thus radiation with wavelengths between the first and second wavelengths will not give rise to a signal in the external circuit.